An ion exchange membrane method is well-known which produces chlorine and a caustic soda aqueous solution by electrolyzing saturated brine by means of a gas diffusion electrode. In this method, an electrolytic cell is divided, by an ion exchange membrane, into an anode chamber equipped with an anode and filled with brine, and a cathode chamber equipped with a cathode and filled with a caustic soda aqueous solution. The electrolysis is carried out by feeding current between the above two electrodes while oxygen-containing gas (oxygen concentration is 100% to 20%) is supplied into the cathode chamber to produce the caustic soda aqueous solution and the chorine in the cathode chamber and the anode chamber, respectively.
The electrolyzing method using the gas diffusion electrode as the cathode enables the reductions of the theoretical decomposition voltage by about 1 V and of the power cost by about 30% compared with those of an ordinary hydrogen-evolving electrolyzing method because no hydrogen evolves on the cathode in the former. Various studies are conducted for bringing the above brine electrolysis using the gas diffusion electrode to the practical use. In this regard, Patent Publications 1 and 2 propose, as a means of further reducing the electrolysis voltage, a method in which a cathode liquid chamber is substantially removed by intimately adhering the gas diffusion electrode to an ion exchange membrane, or the cathode chamber is configured as a gas chamber (this method is referred to as a two-chamber method because the electrolytic cell consists of the anode chamber and the cathode gas chamber). This method advantageously reduces the electric resistance to the lowest limit to maintain the electrolysis voltage minimum because no gap for catholyte exists between the ion exchange membrane and the cathode.
Patent Publication 3 discloses a brine electrolytic cell equipped with a gas diffusion electrode in a cathode chamber in which the electrolysis is conducted while the cathode chamber containing catholyte and oxygen-containing gas is pressurized (three-chamber ion exchange membrane electrolytic cell). In Patent Publication 3, the cathode chamber is pressurized for realizing the intimate contact between the gas diffusion electrode and the ion exchange membrane.